Containers used for the shipping, storing, and delivery of liquids, such as medical or therapeutic fluids, are often fabricated from single-ply or multi-ply polymeric materials Two sheets of these materials are placed in overlapping relationship and the overlapping sheets are bonded at their outer peripheries to define a chamber or pouch for containing liquids. It is also possible to extrude these materials as a tube and to seal longitudinally spaced portions of the tube to define chambers between two adjacent seals. Typically, the materials are joined along their inner surfaces using bonding techniques such as heat sealing, radio-frequency sealing, thermal transfer welding, adhesive sealing, solvent bonding, sonic sealing, and laser welding.
For most applications, the seal formed must be of sufficient strength to withstand the stresses generated by transporting, dropping, or agitating the liquid-filled container. Problems have been encountered with certain materials that do not bond well with themselves or other materials.
Problems have been encountered in forming strong seals in containers having sidewalls made of a layered material. Such a container is described in copending and commonly assigned U.S. patent application Ser. No. 08.backslash.330,717, and is constructed of a multi-layered polymeric material having an outer layer of a polymer blend of styrene-ethylene-butene-styrene ("SEBS") block copolymer (40%-85% by weight), ethylene vinyl acetate (0-40% by weight), and polypropylene (10%-40% by weight) and an inner layer of a polystyrene. The polystyrene layers face one another on the interior of the container and are welded together. However, the bond between the polystyrene layers was not sufficiently strong to allow the container to be centrifuged at useful speeds without bursting and could not be dropped at heights of above six feet without the seals failing.
Some medical containers can contain a relatively large quantity of fluids (1-2 liters), i.e., large volume containers. As the fluid volume increases in the container, so does the hydraulic forces of the fluid. In instances where the containers are filled prior to transporting them to the end user, hydraulic forces can cause the seals' to fail. Similarly, when a medical container of this type is dropped, tearing along the sealed portion can develop when the hydraulic forces exceed the tear initiation force limit of the seal. The tear can propagate along the seam, upon the application of forces well below the tear initiation force, until the container bursts. Thus, a container may not burst upon dropping, but may do so upon handling afterward.
A method designed to improve seal strength is disclosed in U.S. Pat. No. 4,968,624 issued to Bacehowski et al. and assigned to Baxter International Inc. ("Bacehowski"). Bacehowski discloses a medical container having two peripheral seals in horizontal spaced relationship. Sidewalls of the container are fabricated from a multi-ply material with uniform ply thickness across the sidewalls. This structure increases the burst strength of the container. However, Bacehowski does not provide a method for bonding materials that have varying ply thicknesses across the film.
Due to the problems relative to sealing containers manufactured from multi-layered materials, there is a need for creating a suitably strong peripheral seam in such containers.